In a pyroprocess for firstly separating fission products and secondly separating uranium from other materials after metallization of nuclear material in spent nuclear fuel, it is very important for ensuring nuclear transparency and is the recommendation of International Atomic Energy Agency (IAEA) to measure an amount of the nuclear material in each unit process. In order to quickly and precisely measure the nuclear material of various forms under the severe condition of hot cell in which the pyroprocess is proceeded, a neutron counter should be simplified and have high efficiency.
In general, various nuclear fuel materials include a fissionable material which is capable of nuclear fission. A precise detection for an amount of the fissionable material included in the nuclear fuel material is primarily required to produce nuclear fuel which can be burned in a nuclear reactor. The method for measuring the amount of fissionable material is largely divided into a destructive method and a nondestructive method. The destructive method is a chemical analyzing method and has an advantage of good analysis accuracy. However, it is economically disadvantageous since it takes long time to analyze and thus the destructive method has not been generally used. On the contrary, the nondestructive method is an analyzing method by measuring gamma rays or neutrons and has an advantage that it is capable of real time analysis. However, a gamma ray spectrum analysis has been used only to a new nuclear fuel and a method that quantificates Cm which is a major neutron source by measuring neutrons emitted from the Cm and then quantificates U-235 or Pu based on the quantitificated Cm is used for a fissionable material included in spent nuclear fuel. At that time, a highly efficiently neutron detector is needed.
In recent 1990s, there has been developed a neutron detector tube with good detection efficiency. Such detector tube has been mainly utilized in a technology for measuring plutonium content by a neutron multiplication factor measurement method. Particularly in United States, a large number of studies have been made mainly by Los Alamos National Laboratory, etc. Some of them have been commercialized by measurement equipment makers such as Canberra, and IAEA actually and widely utilizes these products in accounting for and control of nuclear material and inspection activity.
However, in the aforementioned neutron detector tube, since the measurement result may be overestimated due to induced fission in a sample and resultant neutron multiplication when there is a high sample density or a much sample amount, there is a difficulty that a multiplication factor for every sample should be known in advance to correct the result. In order to improve the difficulty, there has been suggested a method of measuring with exclusion of multiplicated neutrons using a multiplicity mode device (e.g. JSR-14 available from Canberra) with a high efficiency counter. However, an efficiency of the counter should be more than 40% to use the method and a large number of He-3 tube detectors should be contained to obtain such high efficiency. For example, it has been known that approximately 120 He-3 tube detectors should be contained to obtain the efficiency of approximately 60%. When using such large number of He-3 tube detectors, there are problems that it takes much cost and difficulty in installation and maintenance is increased as a scale of equipment is much increased.
FIG. 1 is a photograph showing a neutron counter, which had been developed and used by Ispara laboratory in Italy and is capable of measuring Pu content in MOX nuclear fuel with a relative error of less than 2%, and in which 125 He-3 tube detectors are contained in the form of four rings and a multiplicity mode is enabled. The counter has a diameter of approximately 96 cm and a weight of approximately 1200 kg but is known to have an efficiency of approximately 50 to 60%. As described above, since such counter requires a large number of He-3 tube detector, its scale is much increased and thus there is a problem that it is not easy to install the counter in a hot cell which is pyroprocessing facility for the spent nuclear fuel and it is also very difficult to maintain the counter.